The Sexually Dimorphic Nucleus of the Preoptic Area (SDN-POA) of the rat hypothalamus is larger in volume in the adult male than in the adult female. Furthermore, the morphological sexual differentiation of this nucleus (in terms of volume) occurs during the perinatal life of the animal. This process has been shown to be influenced by the gonadal steroid environment during early postnatal life. Since there is no sex difference in the size of neurons in the SDN-POA, the larger SDN-POA of the male must contain more neurons than that of the female. Thus, it appears that gonadal steroids during development at least partially determine the ultimate number of neurons which will reside in the SDN-POA. Studies have indicated that neurons inthe hypothalamus become postmitotic prior to birth; how can gonadal steroids increase neuronal number in the male as compared to that of the female postnatally? In the initial study aimed at investigating this question, a specific marker for the presumptive neurons of the SDN-POA was discovered by utilizing thymidine autoradiography. In addition, data indicate that the sexes appear to differ in the gestational age when future SDN-POA neurons are produced. In the proposed studies the SDN-POA will be used as a model system in order to investigate the interaction of the gonadal steroid environment of the perinatal animal and the morphological development of the central nervous system. Thymidine and steroid autoradiography, immunohistochemistry, as well as light and electron microscopy will be used in order to investigate the following questions: 1) Can developing SDN-POA neurons directly be affected by the perinatal gonadal steroid environment? 2) Do sex steroids influence SDN-POA differentiation by affecting neurogenesis, neuronal migration, neuronal death, neuronal organelle formation and/or synaptogenesis? 3) Which (if any) of the neurohormones are involved in the steroid-dependent sexual differentiation of the SDN-POA? The results of this research program will help explain the mechanisms by which the hormonal environment influences a) neuronal growth and development and b) the sexual characteristics of an individual.